Constant envelope FSK/PSK modulations are widely adopted in wireless standards like Zigbee and Bluetooth low energy for their low hardware complexity and better immunity to interference. Conventional I/Q based FSK/GFSK receivers employ frequency demodulation techniques like differentiate-and-multiply, zero-crossing detection and quadrature correlation.
These receivers are power-hungry and employ several analog blocks whose performance degrades with technology scaling. Sliding IF based receivers, although they may partially reduce power consumption, are prone to image rejection issues. Phase domain receivers are becoming a better option for ultra-low power and low-voltage radios.
A phase-locked-loop (PLL), in its locked state, maintains a constant phase difference between the oscillator, normally a voltage-controlled oscillator (VCO), output and a reference signal. In other words, the oscillator in the PLL tracks the frequency of the reference signal. This phase/frequency tracking behavior of a PLL can be used for frequency demodulation, and the control signal to the oscillator can be used directly as a demodulated frequency signal for subsequent processing.
VCO-based phase tracking FSK/GFSK receivers use a PLL demodulator either as a separate block at the end of the receiver chain before analog-to-digital converters (ADC) convert the signal into the digital domain, or integrate it together with other receiver blocks like mixers, low pass filters and variable gain amplifiers. In these approaches, an RF input signal is generally down-converted to a suitable intermediate frequency and amplitude limited before passing it to the PLL for demodulation. The control signal of the oscillator in the PLL is then digitized for digital baseband (DBB) processing.
VCO-based phase tracking receivers suffer from a few architectural issues. First, a PLL demodulator employs a free-running oscillator to lock with an input RF signal. Wireless standards allow RF input signals to have a certain amount of dynamic frequency offset during data transmission. These input frequency deviations along with low frequency drift due to frequency instability of the free-running oscillator drastically affect the carrier recovery during demodulation and can result in a high bit error rate. Thus, Phase tracking FSK receivers may have poor frequency deviation tolerance.
Second, strong interferers pull the oscillator away from the desired carrier frequency which results in poor demodulation performance. This can be partially addressed by using high-order channel-selection filters to suppress the interferers. However, a long delay introduced by the high-order filter degrades the demodulation performance and loop stability.
Finally, a high resolution ADC or a high precision threshold detector is needed to discriminate the FSK data.